(a) Field of the invention:
The present invention relates to an electro-acoustic transducer of the so-called electro-dynamic type for use in, for example, loudspeakers, earphones and microphones, and more particularly it pertains to an electro-acoustic transducer using a planar type diaphragm.
(b) Description of the prior art:
In the past, diaphragms for electro-acoustic transducers as loudspeakers have been formed with such planar type diaphragms as shown in FIG. 1. This diaphragm 1 is constructed into honeycomb structure comprising a honeycomb core 2 formed in planar shape and skin members 3 and 4 each being made of a thin film of material such as fiber-reinforced plastics (FRP) or aluminum (Al) which is applied on each side of a honeycomb core 2. This honeycomb structure has such prominent features as: being light in weight, having a large flexural rigidity, being able to eliminate peak dip of frequency characteristic due to cavity effect which tends to produce in cone type diaphragms, as well as being able to suppress partial vibrations of lower degrees of modes by designing so as to drive the node of a critical partial vibration mode, to thereby issure that the high frequency portion of the reproduced frequency range can be broadened.
More particularly, in the planar diaphragm 1, there are present such partial vibration modes as shown in FIG. 2. Here, the respective numerals in parentheses shown in FIG. 2 represent numbers of the respective degrees of partial vibration modes. For Example, in case a voice coil bobbin 6 is bonded to the node 5 of the second degree resonance mode, and the diaphragm 1 is driven at this node 5, it is possible to suppress as far as the seven degree resonance mode shown in FIG. 2. Therefore, the high frequency region of the reproduced frequency range is broadened.
However, in the known diaphragm 1 of this type, the voice coil bobbin 6 has large diameter to match the increased outer diameter of the diaphragm 1. Thus, there have been the inconveniences that the transducer per se has become large in size, leading to an increase in the manufacturing cost, as well as the drawback that the bobbin 6 has tended to develop mechanical distortions so that the bobbin would contact the magnetic circuit. Also, as shown in an enlarged scale in FIG. 3, the voice coil bobbin 6 has its upper end face bonded integrally to the lower side of the diaphragm 1 by a bonding agent 7 to provide the state that said upper end face of the bobbin 6 is in contact with the lower side of the diaphragm. Thus, the area of contact between the two members is small and accordingly a sufficient strength of bonding cannot be obtained. For this reason, there have been the inconveniences that the diaphragm 1 becomes detached from the voice coil bobbin 6 so that the bobbin 6 comes off due to, for example, degradation of adhesiveness of the bonding agent 7 resulting from use for an extended period of time or to a large output performance.
In view of the foregoing inconveniences of the prior art, there have been proposed of late such electro-acoustic transducer as shown in FIG. 4 which is intended to solve these inconveniences. This recently proposed transducer arrangement is such that a voice coil bobbin 8 having a reduced diameter is coupled to a diaphragm 1 via a conical-shaped drive cone 9 made of such material as aluminum foil. This known structure, however, has the drawbacks such that the drive cone 9 becomes flexed during the operation of the transducer so that it is unable to perform good driving of the diaphragm 1, and also that owing to the small bonding strength of the portion of the drive cone 7 at which the cone is in contact with the diaphragm 1, it is not possible to perfectly eliminate the coming off of the voice coil bobbin 6 from the diaphragm.
Also, in the prior art, the known electro-acoustic transducer as shown in FIG. 4 as an example, has been assembled by constructing a vibration system assembly A comprising, as shown in FIG. 5, a diaphragm 1, a voice coil bobbin 8, a voice coil 10, a drive cone 9, an edge member 11 and a damper 12, and, separately from the assembly A, an outer magnet type magnetic circuit assembly B comprising a pole piece 13, a magnet 14, a yoke plate 15 and a frame 16. Thereafter, the vibration system assembly A is attached to the magnetic circuit assembly B, for completion. During this construction, the vibration system assembly A is secured to the frame 16 after being centered of its position relative to the frame 16, and the peripheral edge portion of the diaphragm 1 is conjugated to the open end edge of the frame 16 via its edge portion 11 and a gasket not shown.
However, in such known manufacturing process as stated above, the vibration system assembly A is inserted into the mangetic circuit assembly B while centering or aligning the axes of the diaphragm 1 and of the voice coil bobbin 8 simultaneously. Thus, this centering is not performed with good accuracy. As a result, there is left uneven portions of clearance between the voice coil 10 and the yoke plate 15. In the worst case, these two members have portions at which they contact each other.